Although the content and nature of teachers' practical and theoretical knowledge has continued to evolve since the mid to late 1970s, current conceptualizations of teacher knowledge do not adequately reflect the knowledge and skills teachers need to successfully navigate a technologically rich educational environment. This article summarizes traditional and contemporary organizational schemes for teacher knowledge and argues that lacking from our understanding of teacher knowledge is a new domain of expertise: pedagogical technology knowledge. The theoretical development of this new knowledge base includes consideration of instructional and classroom management issues, the interplay between content and technology, and pedagogically appropriate conceptions and uses of technology. The theoretical development of this knowledge base has the potential to inform practice, frame instruction, and stimulate the thinking of both teachers and researchers.